This invention relates to a turbocharger including an electric machine integrat with it.
Internal combustion engines, particularly diesel engines, are commonly turbocharged to enhance performance. The turbocharger utilises energy remaining in the exhaust gases from the engine to drive a turbine which is directly coupled to a compressor. The compressor compresses inlet air which is fed to the intake manifold of the engine to increase the density of the air charge entering the cylinders. In addition, an intercooler is often installed between the compressor and the manifold to remove the heat generated from the compression of the charge air. This reduction in temperature helps to further increase the density and hence the mass of air entering the engine.
The amount of compression of the air provided by the turbocharger is related to the speed of the rotation which in turn is related to the amount of exhaust gas which is dependent upon the speed of the engine. Consequently, at low engine speeds, the turbocharger cannot provide air at a high enough pressure to the engine so that the boost to the engine is lower and the engine does not accelerate very quickly. As the engine and hence the turbocharger speed up, the pressure of the air is increased and the power of the engine is boosted. This delay before the turocharger begins to take effect is known as xe2x80x9cturbo lagxe2x80x9d. The turbo lag can be reduced by increasing the size of the turbine thus providing more power to the compressor at lower speeds. However, at higher engine speeds it is likely that it will become necessary to reduce the turbine power so as to avoid over-speeding the turbocharger and/or over-boosting the engine. This is conventionally achieved with the use of a xe2x80x9cwastegatexe2x80x9d. The wastegate works by bypassing excess exhaust gases past the turbine. The wastegaste is usually controlled by the compressor discharge pressure which provides an indication of the speed of rotation of the turbocharger.
In order to optimise a turbocharger to provide satisfactory performance during steady state operation but also provide adequate transient response, e.g. during acceleration, it is necessary to make a number of compromises. These compromises can result in a reduction in efficiency of the turbocharger and hence the engine, reduced power during steady state operation and poorer aerodynamic efficiency of the turbocharger.
Various ways have been proposed for assisting the turocharger in overcoming its limitations at low engine speed Among these are providing an electric motor which is coupled to the turbocharger to provide additional power to the drive the compressor at low speeds. In addition, at high speeds, the motor can act as a generator to bleed power away from the turbocharger to prevent the turbocharger from over-speeding.
Typically hybrid turbochargers involve integrating an electric motor between the turbine and the compressor wheels which are provided at each end of the turbocharger. However, the motor being in close proximity to the turbine and compressor wheels results in it being subjected to extremes of temperature and thermal cycling. This can lead to a reduction in the insulation life and an increase in the risk of heat soaks which can lead to irreversible losses. In addition, in the prior art it is necessary to provide special bearing arrangements, see U.S. Pat. No. 5,857,332, as well as special provisions for cooling, see U.S. Pat. No. 5,605,045. In addition, these constructions lack the potential for scaling to higher power levels such as those required for heavy duty applications such as in ships and locomotive engines.
Throughout this specification, references to motors include motors which may act as generators.
Therefore, according to the present invention there is provided a turbocharger assembly comprising: a turbocharger and a motor coupled to the turbocharger
The present invention provides a turbocharger with a motor assembly which can be easily added to or removed from the turbocharger. The motor is preferably coupled to the turbocharger using a flexible coupling arrangement which provides added integrity and provides for reduced rotor dynamic coupling between the turbocharger and motor shafts. Furthermore, the arrangement of the present invention can easily be retrofitted to existing turbocharger systems with minimal changes to the interfaces. In particular the motor can be attached to the compressor side of the turbocharger without adversely affecting the air induction system so as to minimise any pressure losses at the inlet of the compressor. Furthermore, because of the convenient positioning of the motor assembly, the system is easily accessible and so repair or replacement of parts is much easier resulting in a minimum downtime in case of failure of the motor.
The motor assembly is preferably provided by a permanent magnet axial flux motor. A typical axial flux motor comprises a generally disc-shaped stator mounted between two concentric rotor discs. The stator and the rotor discs each have an opening to receive a central spindle or shaft. A plurality of equi-angularly spaced permanent magnets are mounted, by means of adhesive or otherwise, close to the outer edges of the rotor discs, facing the stator. A side view of an axial flux motor with a rotor disc and magnets is shown in FIG. 1 of the drawings. Cooling airflow is drawn into the machine and is pumped outwards by the rotor. The stator comprises a generally disc-shaped substrate onto which stator windings have been etched in a copper sheet using known techniques. Alternatively the stator may comprise windings embedded in the substrate.
This type of motor provides high-speed capability and high efficiency. As the motor is self-cooled using air, it is capable of being easily adapted to the volume and aspect ratio requirements of a typical turbocharger. This provide a turbocharger assembly which is much more compact and suitable for mounting on existing turbochargers or in spaces limited to the space of a conventional. Furthermore, such motors are available in a broad range of power levels including up to the 100 kW to 250 kW power levels required for ships and locomotive engines.